(Reuters) -Union members who assemble Boeing's ( BA ) fighter jets in the St. Louis area have "overwhelmingly voted" to reject the company's contract offer on Sunday.

Boeing's ( BA ) proposal, which was sent on Tuesday to more than 3,200 members of the International Association of Machinists and Aerospace Workers (IAM) District 837, included a 20% general wage increase over four years and a $5,000 ratification bonus, as well as more vacation time and sick leave.

"The proposal from Boeing Defense fell short of addressing the priorities and sacrifices of the skilled IAM Union workforce," the IAM union said.

Boeing ( BA ) did not immediately respond to a Reuters' request for a comment.

The current contract expires on Sunday following which there is a seven-day cooling off period before a strike would begin, the union added.

Boeing's ( BA ) defense division is expanding manufacturing facilities in the St. Louis area for the new U.S. Air Force fighter, the F-47, after it won the contract earlier this year.